[composer] Migrate metrics command to Symfony runtime

[coisa]

Summary

Migrate the metrics command from the legacy Composer BaseCommand runtime to the new Symfony-command runtime proposed in #199.

This is one command-level subissue for the incremental Composer decoupling plan. It keeps the migration small enough to validate through both command entrypoints before moving to the next command.

Context

Parent issue: #199
Architecture discussion: #197

Current command class:

• FastForward\DevTools\Console\Command\MetricsCommand

Migration Scope

• Convert the real command implementation to extend Symfony\Component\Console\Command\Command instead of Composer\Command\BaseCommand.
• Remove assumptions that depend on Composer IO or BaseCommand behavior from the real command.
• Ensure the Composer capability exposes this command through the new ProxyCommand adapter once migrated.
• Preserve the existing command name, aliases, description, help, arguments, options, output behavior, and exit codes.
• Keep command-specific behavior in the real Symfony command or a handler, not in the Composer proxy.

Command-Specific Notes

Risk level: Medium

Runs PhpMetrics and report-summary behavior. Preserve target/junit/exclude behavior.

Acceptance Criteria

• MetricsCommand no longer extends Composer\Command\BaseCommand.
• The command is available through bin/dev-tools metrics --help.
• The command is available through Composer plugin discovery in the composer-plugin-consumer fixture.
• The Composer-facing command is a proxy when this command has been migrated.
• Existing tests for MetricsCommand are updated to exercise the Symfony command runtime.
• At least one regression verifies that the Composer proxy can execute or display help for metrics.
• No user-facing option, argument, alias, or help text is intentionally removed.

Non-Goals

• Do not extract this command into a separate package in this issue.
• Do not build dev-tools-shim in this issue.
• Do not migrate unrelated commands in this issue unless required by shared infrastructure.